How Does a Bicycle Helmet Work?
A helmet reduces the peak energy of a sharp impact. This requires a layer of stiff foam to cushion the blow by crushing. Most bicycle helmets do this with expanded polystyrene (EPS), the white picnic cooler foam. Once crushed, EPS does not recover. Another foam, expanded polypropylene (EPP), does recover, but is much less common. A stronger EPS called GECET appeared in 1992 and is widely used now. Another foam called EPU (expanded polyurethane) is used in Taiwan. It has a uniform cell structure and crushes without rebound, but is heavier and requires a manufacturing process that is not environmentally friendly. The spongy foam inside a helmet is for comfort and fit, not for impact. The helmet must stay on your head even when you hit more than once–usually a car first, and then the road. So it needs a strong strap and an equally strong fastener. The helmet should sit level on your head and cover as much as possible. Above all, with the strap fastened you should not be able to get the h
